Deflection controlled tube



June 5, 1962 K. SCHLESINGER DEFLECTION CONTROLLED TUBE Filed March 5l.1958 RJ swam 2 sheets-sheet;

INVENTOR. lem' June 5, 1962 K. scHLl-:slNGER 3,038,101

- DEFLECTION CONTROLLED TUBE PMM/ @MJU United States Patent O 3,038,101DEFLEC'HQN CNTRQLLED TUBE Kurt Schlesinger, La Grange, lll., assigner toMotorola, luc., Chicago, Ill., a corporation of Illinois Filed Mar. 3l,1958, Ser. No. 725,270v il Claims. (Cl. 315-22) This invention relatesto a new and improved crossedeld modulation system for use incathode-ray tubes to modulate Iand focus a beam and thereby control theintensity of a spot on a viewing screen. y

Cathode-ray tubes in present use include an electron gun having a smallarea or point cathode modulated by an adjacent grid, which together witha first anode converges the beam into a crossover. A lens is provided toimage the crossover on a viewing screen so that a spot is formedthereon, and the grid-modulation controls the intensity of the spot.y

Some generally recognized standards for a good electron gun are that itshould provide high beam current and resolution, good halftonerendition, high modulation sensitivity, and minimum interaction betweencontrol voltage and spot size. For many applications, and particularlythose in which the control voltage is developed by transistor circuits,it is desirable for the control characteristic to permit modulation withlow voltages of the order of 12 volts or less. It has been found thatcathode-ray tubes with conventional grid-modulation control cannot meetall of the above general requirements; there has to be a compromisebetween brightness and resolution.

Accordingly, one object of this invention is to provide a new andimproved system for modulating and focusing an electron beam.

Another object of the invention is to provide a cathoderay tube with anew and improved intensity control system which has high modulationsensitivity at low beam voltages.

Still another object is to provide an improved deflection control usingbeam interception in which scattered electrous resulting from suchinterception are collected separately in order to reduce halation on thescreen.

A feature of the invention is the provision of a cathoderay tube with alarge area cathode providing a high current beam which is concentratedby a focusing modulator into a small image-defining aperture whichpresents a stable size and shape with controlled illumination.

Another feature of the invention is the provision of a focusingmodulator assembly having electrostatic electrodes generatingcrossed-fields simultaneously providing strong focusing elds and weaksuperimposed deecting fields, thereby permitting intensity control of ahigh current electron beam with low control voltages.

Another feature of the invention is the provision of a crossed-fieldmodulator including a slotted-cylinder unit providing two electricfields in a cavity; one field being coaxial for Ifocusing an electronbeam, and the other being transversal for deflection modulation of thebeam.

A further feature of the invention is the provision of a focusingmodulator including two separate bifocal fieldgenerator systems arrangedin quadrature along the tube axis, with each system having fourpole-pieces shaped .as

rectilinear hyperbolae, and with the first system receiv' ing themodulating signal along its positive focal plane.

A further feature of the invention is the provision of a new andimproved deflection modulation system having a beam intercepting anode,and wherein scattered electrons and secondary emission from theinterceptor anode are drawn backward to a collector anode which ispositively biased.

A still further feature is the provisionl of a focusing modulator havinga post-focuser at its exit end including rice a concave profileimmediately following an intercepting aperture for concentrating thelbeam in the presence of post-accelerating elds.

The invention is illustrated in the accompanying drawings in which:

FIG. 1 shows a cathode-ray tube including a crossedeld modulator inaccordance with one embodiment of the invention;

FIG. Z is ya slightly enlarged cross-sectional view of the modulatorstructure shown in FIG. 1; l

FIG. 3 shows a printed ceramic construction forrthe modulator of FIG. l;

FIG. 4 is a diagram of a control circuit suitable for use in thecathode-ray tube shown in FIG. 1;

FIG. 5 is a curve illustrating the control characteristic of themodulator of FIG. 1; l

FIG. 6 shows a modification of the crossed-field modulator structureshown in FIG. l; and

FIGS. 7 and 8 show a cathode-ray tube including a focusing modulator inaccordance with another embodiment of the invention.

The invention is a modulation system for cathode-ray tubes in which ahigh current beam of electrons passes through a focusing modulatorincluding adjacent electrode groups, each having a plurality ofpole-pieces bounding a common cavity. The pole-pieces generate crossedelectric fields in the cavity which simultaneously exert focusing anddeflecting action on the beam.- In a slottedcylinder embodiment, thepole-pieces have a semi-cylindrical shape. In another embodiment, thepole-pieces are shaped as rectilinear hyperbolae. In either case, two ormore groups of pole-pieces are closely spaced coaxially to form thecommon cavity in which deflection and lfocusing take place. The radialdistance from the axis of the cavity to the active faces may vary fromone group to another. A form of insulating material is provided tolocate the pole-pieces in the proper positions, and the pole-pieces maybe provided as coatings of conductive material on the form. In theslotted-cylinder embodiment, the focusing action is provided by a fieldwhich acts coaxially of the modulator while deflecting action isprovided by fields which act transversely and are, therefore, crossedwith respect to the focusing field. In

ythe hyperbolic embodiment, simultaneous deflecting and focusing actionis obtained with a single pair of electrostatic 4-po1e systems arrangedin quadrature along the same axis, with only the rst system beingconnected to the modulating signal. In both embodiments, an aperturedinterceptor anode is positioned at the exit end of the cavity. The beamof electrons passes through the cavity with little loss since it isconverged to form a sharp, small crossover right at the interceptor. Thedeflecting field causes part of the beam to impinge yon an edge portionof the interceptor, while the remainder of the beam passes through theaperture and thus varies with the modulating signal. Electrons which areintercepted tend to scatter, and this may cause undesirable halation ofthe picture background. To prevent this, a collector anode is positionedbehind the interceptor to pull the scattered electrons backward. Thecollector anode may be a separate apertured disc, or may even be one ofthe groups of pole-pieces if the cavity therethrough is made to have asuiiiciently small diameter at its exit end.

In FIGS. 1 and 2 there is shown part of a cathode-ray tube having afocusing modulator of the slotted-cylinder type. Electrons are emittedfrom the large area cathode 1t). The electrons are accelerated away fromthe cathode 1-0 into the modulator cavity 33 where they are focussedinto a crossover. This crossover is placed right in the spot definingaperture 44 in -the interceptor anode 43. The beam is deflected inresponse to a modulating signal applied to half of the slotted-cylinderso that the portiony of the beam passing through aperture '44 is' con-`trolled. A positively biasedcollector plate. 5.1 havingy f -t-entialstoeach half vof the cylinder, a transverse Afield: l

an aperture l52y is positioned behind the interceptor with they edgev ofaperture 52 closely spaced kfrom they intercepting edge houndingaperture44. in order to` pull back yscattered'electrons which tend to causehalation 'on the Screen. f

The beamdiverges after passing through the aperture 20 by theinteractionof a concave meniscus 45. at low :voltage andthe main llen-sbarrel` 17 at higher voltage.y

f 44, and is re-converged into an image-forming 'section l Since theinterceptor presents `a beam of deiinite shape potentialv electrostatic,un-potential electrostatic, ,mag-l ynetic, and others. The image formingsection is pref- .erablyl of large; diameter to reduce sphericalaberrations. vThein'xaging lensintyv also include asecond anode 18 i lwhich is preferably acoating -18 .of conductivematerial v 1 kon ith:lwall 19 ofl the tube.: f .This liens vsection is adjust- .ed to focusthe image of the aperture 44 into a well: de-i i fined spot ontheviewing screen .21. f f

. The cathode ;10.is provided with a comparatively large emitting4surface. 26, and since beam current. is propori tional to cathodeareahigh beam currents rmay therebyv be provided.` .The -cathodeisurface26 is concave ito preiocusing end-plate 41. provide a ield whichpreconwith varying illumination, the image forming section 1 may be anyof several conventional types, ysuch as ybiisv established between thehalves'. Thus, the bcamis deflected `slightly from the centcr of theaperture 44.

lThis isy accomplishedv by moving the arm .iliileither above or belowthe center position. Thenr when a ymodulating Signal isappliedtoene-half of the unit, theerieet is to f f shift the position of thebeam focus transverselyl in rcl sponse tothe variations ofthe signal. vf

; The picture displayy from a'givenv signal will be a positive ornegative image, depending on the oit-center position of the focusvvithrespect to aperture ,44. Accordzingly, the `crossed-held modulator tubeis capable kof handling picture signals of either polarityl by reversingthe l bias across the cavity. 'Since' thev beam-voltage .in the :cavityisrcomparatively slow, thev deflecting field can ,be` weak, andeiiicient-modulation canbehad at aloW signall .level of theorder. of 12volts'and less. A control .char- :actenistic for a .modulator in.accordance vwith FIG. 1 is Shown in FIG. 5 `to illustrate the; highmodulation sensi.'r c fil/ity `and the positive-negative picture controldescribedA labovepmf.. l

, The followingdimensions for :a :cathode-rayA tube built f n inaccordance with FIGS'. l and-2 arey giveny by yway of f facilitatefocusing. vThe collimating aperture 27 andA the verges/the *beamprior toitsentry into the modulator cavity 33.r .Relative placement of cathode10, collimator d2, 30

and end-plate 41 maybe chosen such thaty thev collimator lbiasoptirnizesatl ground potential.` y

v The modulator vlrincludesthree pairs of pole-pieces:r .l

glass, or other insulating material. The inner faces 8l and 82 of thepole-pieces 38 and 39 respectively, and the manner in which they boundthe slotted cavity 33, may be seen clearly here.

The poles 34-39 are shown in FIGS. l and 2 as halfrings of metal, butmay also be constructed by applying coatings of conductive re paint toan insulating form such as is illustrated in FIG. 3. The pins 132,preferably of Kovar, extend through the semicylinders 130 and makecontact with the inner surfaces 3439. In either type of construction,the pole-pairs are separated by narrow insulating gaps 40.

The circuit diagram of FIG. 4 illustrates how electric potentials may beapplied to the elements of a modulator structure n accordance with FIG.l. A potential of about 250 volts may be applied to the two end-plates41 and 51 in parallel. A slightly lower potential is applied to theinterceptor 43 and also to the pole-pairs 34-35 and 38-39. A low voltagebias is applied to the central pole-pair 36--37, and the latter pair mayeven be grounded. A structure of this type provides a non-linear spatialpotential distribution which exerts strong focusing action with lowspherical aberration, especially if this spatial potential variation isparabolic. By judicious choice of pole-piece length and bias, the focallength of this modulatormay beadjusted so as to focus the beam into asharp `crossover right at the spot dening aperture 44 of the interceptor43.

When opposing pole-pieces are at the same potential, the field `formedthereby has circular symmetry. Therefore, the crossover is centered withaperture 44 and most of the beam passes through it. This symmetricalfield distribution exists when the arm 100 on potentiometer 102 is atthe center position. By applying different poiilustration'and 'arel notintended to limit the lscope of the y invention in any lway:

Separation lbetween cathode 10 and grid 12 Separation between poles344-35, 36--37, 38--39 i 0.030 Transverse spacing betweenl pole-pairs34: and 35;, l f 36-and k3'7, .and 38-and 39 f 0.030

Diameter ofprefocusier aperturel 42 0.100 Diameter of grid aperture 27 lf 0.150

Diameter of collector aperturev 52 0.060

-Diameterof interceptor aperture 44 0.025 Diameter of emitting surface26 0.060 Diameter of cathode '10 0.125 DiameterE of meniscus 45-.; c0.250 Radius of meniscusl 4'5 l 0.125 Diameter of cavity 3'3 '.f.'.0.250 Length of pole-pieces 34-35 4/16 Length of pole-pieces 364:9 5716Thickness of end plates 41 and 51 0.010 Thickness of disc 43 0.062

The following circuit values are given for the circuit shown in FIG. 4only by way of illustration:

Such a cathode-ray tube has been found to have the followingcharacteristics:

Modulation voltage 12 volts and less.

Maximum transconductance 150 microamperes per volt.

Maximum beam current G-1200 microamperes.

Highest voltage needed for modulator 250 volts. Resolution Better than400 lines.

In FIG. 6 there is shown a modification of the modulator of FIG. 1 inwhich the cavity diameter tapers down towards the exit. In the taperedstructure shown here, the faces 91 of pole-pair 61--62 at the cavityexit have a diameter `which corresponds roughly to that of edge 52 shownin FIG. 2. Pole-pair `61--62 is maintained at a slightly higherpotential than the interceptor 71 to pull Inch i i .030 Separationbetween grid 12 and prefocuser 41 f 0.100 f sessant' back scatteredelectrons, and thus no separate collecting structure is required.

A cathode-ray tube provided with a focusing modulator in accordance withanother embodiment of the invention is shown schematically in FIGS. 7and 8. It may be seen that in this embodiment the modulator 14) includestwo adjacent quadrapoles 138 and 139 on a common axis and forming acommon cavity 153. The active yfaces of the ypole-pieces 141-143 are ofhyperbolic shape. The polepieces are held in position by a commoncylinder 149 of insulating material such as lava, as may be seen in FIG.8. Slots 151 are provided in the cylinder 149 to locate the edges of thepole-pieces. A broad beam of electrons from the large area cathode 10passes through the composite system, and will converge to a crossover atthe exit end of cavity 153 when .proper bias is applied to thepole-pieces. An apertured interceptor 43 is placed at the eXit of thecavity. A collector anode 51 is interposed between the interceptor 43and the cavity.

The action of the modulator 140 is somewhat different from that ofmodulator 13 of FIG. l, but there is an overall similarity inconstruction and the beam passing through the cavity is simultaneouslyfocussed `and deiiected by crossed-fields. the cavity 153; one exertinga positive focusing action, and the other exerting -a negative focusingaction. Voltages are applied to each quadrapole in a balanced-to-anodefashion, as shown at X and Y in FIG. 7. As a result, the average beamvelocity stays constant throughout the system. Moreover, experienceshows that the potential difference -and length of the respectiveelectrodes are inversely proportional. If used independently, each quarapole would tend to produce a line focus. It has been found, however,that by employing only one pair of quadrapoles with the polarities ofone rotated 90 with respect to the other, it is possible to obtain anoverall positive focusing action, and under certain conditions of biasthis system can produce a point focus. The conditions for obtainingpoint focus can be described by the equation Where f1 is the focallength of the first quadrapole 138, f2 is the -focal length of thesecond quadrapole 139, and a is the distance between them. The systemcan be biased to pass the beam through the narrow aperture 44, and thefocus is then shifted transversely by a modulating signal applied to oneof the poles 142 and 144.

The potentials required to provide this simultaneous focusing anddeflecting action are applied to the various elements of the modulatorby the circuit shown therewith in FIG. 7. The prefocuser 41 andcollector 51 are placed at about 200 volts, and the interceptor 43 isplaced at a VSomewhat lower potential to insure that scattered electronsare collected. Opposite pole-pieces within each quadrapole system areplaced at the same potential, with .the pairs 141--143` and 146--148being positive with respect to beam voltage, and pairs 142-144 and 14S-147 being negative with respect to beam voltage. The pairs of likepolarity in :groups 138 and 139 are rotated 90 with respect to eachother in order to provide the composite crosSed-ield effect previouslydescribed.

The modulating signal is applied lfrom the signal source 155 topole-piece 144 to initiate deflection modulation. VIt is important toapply the signal to one of those electrodes in quadrapole system 138which is negative with respect to beam voltage so that the firstquadrapole system receives the signal along its positive focal plane. Ifthis is done, the beam -Will 4move in a plane for which the secondquadrapole system is divergent, its corresponding polepieces beingpositive with respect to `anode voltage, as shown. Under theseconditions, the second quadrapole 139 lprovides additional defiectioneven though it is not directly connected to the signal source.

Each quadrapole generates two fields in It is apparent from theforegoing description that the crossed-field modulation system of theinvention provides a high current gun yielding a spot of denite size andshape. This system has a nearly linear modulation characteristic, and itruns on very low signal voltage. Consequently, a tube provided with sucha crossedeeld modulator may be operated in a television receiver wtih alow level of video signal, and is especially suited vfor use withtransistorized circuits which provide signals of only 12 Volts or less.The modulation off the beam by partial interception has little effect onspot size. Spurious halation by scatter electrons can be eliminated bythe provision of a separate collector anode in back of the interceptinganode. A tube having a control system of this type is capable ofdisplaying positive or negative modul-ation. This 1oiiers the practicaladvantage of handling signals of either polarity with out the need for aseparate inverting ampliiier.

I claim:

1. In a cathode-ray tube including a viewing screen, and an electronsource for directing a beam in a predetermined path against the screen,a crossed-field modulation system for controlling t-he intensity of aviewed spot formed by the beam on the screen, said system including incombination, a beam intercepting anode located between the electronsource and the screen and having a spot defining aperture in alignmentwith the predetermined bearn path, a plurality of adjacent electrodegroups located between the electron source and said intercepting anodeand each including a plurality of electrostatic poles positioned onopposite sides of the predetermined beam path, said poles havingtransversely curved active faces extending longitudinally and defining acommon cavity for passage of the beam, circuit means having a iirstportion for applying different biasing potentials to said poles toestablish a focusing field in said cavity for converging the beam into acrossover at said edge of said intercepting anode, said circui-t meanshaving a second portion for applying a modulating signal to certain ofsaid poles for superimposing a transverse -deiiecting field on saidfocusing field which is effective to deiiect said beam between oneextreme condition wherein the beam is directed at said spot definingaperture and another extreme condition wherein substantially the entirebeam is directed on said beam intercepting anode about said aperture,whereby the modulating signal is effective to control the portion of thebeam passing through said aperture and striking said screen, therebycontrolling the intensity of the viewed spot.

2. In a cathode-ray tube having a viewing screen, and an electron sourcefor directing a beam in a predetermined path against the viewing screen,a crossed-field modulation system as defined in claim l having at leastthree adjacent electrode groups each including two opposed poles, saidpoles having active faces of semi-cylindrical [shape forming a slottedcavity, and said first portion of said circuit means applying a lowerpotential to the intermediate pole-pair than to the other two to providea non-linear spatial potential distribution exerting strong positivefocusing action.

3. In a cathode-ray tube including a viewing screen and an electronsource for directing a beam in a predetermined path against the viewingscreen, a crossed-held modulation system as defined in ciaim 1 havingtwo coaxially adjacent electrode groups arranged sequentially inquadrature and each including four poles, said poles having active facesshaped as rectilinear hyperbolae, with the biasing potentials appliedthereto by said first portion of said circuit means being selected toprovide a composite crossed-field exerting positive focusing action, andwith the rst electrode group in the sequence receiving the modulatingsignal along a `focal plane for which the second electrode groups isdivergent.

4. In a cathode-ray tube including a viewing screen, and an electronsource yfor directing a beam in a predetermined path -against theviewing screen, a crossed-field modulation system as defined in claim 1with one of said poles in the electrode group positioned nearest to saidbeam intercepting anode having a portion closely spaced from the surfaceon said interceptor anode impinged by the deflected beam, and with thepotential applied to said closely spaced pole-portion by said circuitmeans being more positive than the potential of said interceptor anode,whereby electrons which are scattered from said interceptor anode areseparately collected by `said pole-portion.

5. In a cathode-ray tube including a viewing screen, and an electronsource for directing a beam in a predetermined path against the viewingscreen, a crossed-field modulation system as dened in claim l andfurther including a collector anode interposed between said beamintercepting anode and the electrode group positioned nearest thereto,said collector anode having a portion closely spaced from a surface onsaid beam intercepting anode impinged by the detlected beam, and withsaid circuit means having a portion for biasing said collector anodepositively with respect to said beam intercepting anode for separatelycollecting electrons which are scattered therefrom.

6. In a ctahode-ray tube having a collimator and a cathode for emittinga high current beam of electrons in a predetermined path, a focusingmodulator including in combination, an interceptor anode having a spotdefining aperture therein of a diameter smaller than the diameter of thebeam emitted by said cathode, a multiple electrode structure positionedbetween the cathode and said interceptor anode, said electrode structureincluding a least three closely spaced electrode groups each including apair of semi-cylindrical electrostatic pole-pieces defining a commoncavity for passage of the beam, a collector anode between saidinterceptor anode and the electrode group at the beam exit end of saidcavity, said collector anode having an aperture aligned with said imagedefining aperture and larger than said image dening aperture, circuitmeans for applying different predetermined biasing potentials to saidpole-pieces thereby establishing in said cavity a focusing field havinga non-linear spatial potential distribution exerting strong focusingaction for converging said beam at the position of said interceptoranode, said circuit means having a portion for applying a modulatingsignal to half of said pole-pieces for superimposing a transverse fieldon said focusing field providing a composite field of variabletransverse asymmetry, with such composite field being effective todeflect the beam between one extreme condition in which the beam isdirected against said interceptor anode and another extreme condition inwhich the beam is directed through said aperture therein, therebyproviding deflection intensity control, and said circuit means having afurther portion for biasing said collector anode positively with respectto said interceptor anode for drawing scattered electrons back from saidinterceptor anode to said collector anode.

7. In a cathode-ray tube, a focusing modulator as defined in claim 6 andfurther including an apertured prefocusing anode between the collimatorand the electrode group at the beam entrance end of said cavity, andsaid circuit means applying a positive potential to said prefocuseranode for preconverging the beam before entry into said cavity.

8. in a cathode-ray tube, a focusing modulator as defined in claim 6with said circuit means including a variable control for adjusting thebiasing potentials applied to said pole-pieces, said control beingadjustable to invert the sense of modulation for converting between apositive and a negative picture.

9. A focusing modulator for use in a cathode-ray tube having acollimator and a cathode for emitting a high current beam of electronsin a predetermined path, said modulator including in combination, aprefocuser anode adjacent the collimator and having an aperture thereinaligned with the predetermined beam path, a multiple electrode structureadjacent the exit side of said prefocuser anode and including two groupsof electrostatic polen pieces arranged in sequence and defining a commoncavity aligned with the predetermined beam path, each of said groupscomprising four pole-pieces arranged in opposed pairs about thepredetermined beam path and having active faces shaped as rectilinearhyperbolae bounding said cavity, a collector anode at the exit end ofsaid cavity and having an aperture aligned with the predetermined beampath, an interceptor anode closely spaced from the exit side of saidcollector anode and having a spot defining aperture therein of adiameter smaller than that of the beam emitted by said cathode andlarger than that of said aperture in said collector anode, said spotdefining aperture being aligned with the predetermined beam path, andcircuit means having a first portion for applying predetermned biasingpotentials to said pole-pieces with said opposite pole-pairs in each ofsaid groups being of opposite polarity, one of said groups being rotatedwith respect to the other for providing a composite crossed electricfield in said cavity for focusing the beam at the position of saidinterceptor anode, and said circuit means having a second portion forapplying a modulating signal to one of said pole-pieces in said firstgroup for superimposing a dellecting field on said composite fieldeffective to deliect the beam between one extreme condition in which thebeam `is directed substantially entirely against said interceptor anodeand another extreme condition in which the beam is directed lthroughsaid spot defining aperture, thereby providing deflection intensitycontrol.

l0. In a cathode-ray tube, the combination including, a large areacathode for emitting a high current beam in a predetermined path, aninterceptor anode spaced from said cathode and having a spot definingaperture smaller than the diameter of the beam emitted 'by said cathode,focuser means lbetween said interceptor anode and said cathode forfocusing said high current beam into a crossover at said spot definingaperture, said focuser means including a multiple electrode structureproviding a composite focus ing field having a variable transversecomponent for deecting the entire focused beam in one lateral directionsubstantially entirely out of said aperture and onto said interceptoranode about said aperture, means for providing modulation potential atsaid electrode structure effective to vary the deliection of the beamwith respect to said spot dening aperture and said interceptor anodeabout said aperture so that the total beam current supplied to saidinterceptor anode and said aperture therein is substantially constantand the portion of the beam current which passes through said aperturevaries from a cut-oli Value to a maximum value, an image forming lensspaced from the beam exit side of said interceptor anode, and concavepost-focusing means at the exit side of said spot defining aperture lforconcentrating the beam before it enters said image forming lens.

11. In a cathode ray tube, the combination including, a large areacathode for emitting electrons in a predetermined path, an interceptoranode spaced from said cathode and having a spot defining aperturetherein, electrode means positioned between said interceptor anode andsaid cathode, said electrode means including a plurality of polesproviding a composite electric :field having both axial and lateralcomponents in the path of the electrons, said poles of said electrodemeans being positioned so that said field is effective to focuselectrons from said cathode into a crossover at said spot definingaperture and to deflect substantially all of such electrons laterallyaway from said spot defining aperture and onto said interceptor anodeabout said aperture, and means for applying modulation potential to saidelectrode means to vary the deection of the electrons with respect tosaid spot defining aperture to thereby control the portion of theelectrons which passes through said aperture, said large area cath odeproviding substantially constant electron current to said interceptoranode and through said aperture therein, with the portion of theelectron current passing through said aperture varying yfrom a Ieut-offvalue to a. maximum value.

References Cited in the le of this patent UNITED STATES PATENTS 10Sziklai Feb. 26, 1952 Smith Dec. 27, 1955 Williams Sept. 24, 1957McNaney Oct. 29, 1957 Kompfn-er May 13, 1958 Laerty Sept. 16, 1958 LogueApr. 28, 1959 Beam May 26, 1959 Glaser Dec. 29, 1959

